This study examines the nature and significance of naturally occurring mutations in the human immunodeficiency virus (HIV) using molecular and cell biological approaches. A unique clinical population will be the major focus of this work. This population includes a homosexual male who transmitted HIV infection to three children via blood transfusion. Isolates of HIV from each of these individuals will be obtained every six months. Primary or secondary peripheral blood mononuclear cell cultures infected with each of these virus isolates will be established, and high molecular weight total cellular DNA prepared. Recombinant DNA clones of the HIV provirus will be isolated from these DNA samples, and analyses utilizing detailed restriction enzyme mapping and nucleotide sequencing will be performed, focusing on the envelope gene of the virus. First, an analysis will be performed on multiple different clones dervied from single virus isolates to determine the level of homogeneity of the virus population within a given individual at one time. Second, clones obtained from isolates from the same individual at different times will be examined to determine the rate of mutation. Third, clones dervied from isolates of each of the four individuals in the common source outbreak will be analyzed and compared to determine the types and rates of mutations of a common progenitor virus in different individuals. The significance of these mutations with respect to possible viral evasion of host immune responses will be examined as well. An improved quantiative assay will be developed by plaque assay for HIV; it will be adapted to provide an improved method for measuring neutralizing antibody responses. This will be used to examine neutralizing antibodies obtained from a given individual 1) towards the virus present at the same time the serum was obtained in that individual, 2) towards virus present in the same individual before and after obtaining the serum sample, and 3) towards virus present in other individuals involved in the common source outbreak and individuals infected with less closely related strains of HIV. These studies should provide important data for the treatment or prevention of this infectious disorder.